MaskingMediaSource needs to resolve the prepare position set for a MaskingPeriod
while the source was still unprepared to the first actual prepare position.
It currently assumes that the period-window offset and the default position is
zero. This assumption is correct when a PlaceholderTimeline is used, but it
may not be true if the real timeline is already known (e.g. when re-preparing
a live stream after a playback error).
Fix this by using the known timeline at the time of the preparation.
Also:
- Update a test that should have caught this to use lazy re-preparation.
- Change the demo app code to use the recommended way to restart playback
after a BehindLiveWindowException.
Issue: #8675
PiperOrigin-RevId: 361604191
